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CHINA FOUNDRY Vol.6 No.1
Numerical simulation of low pressure
die-casting aluminum wheel
*Mi Guofa1, Liu Xiangyu2, Wang Kuangfei1, Fu Hengzhi1
(1. School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, P. R. China; 2. Department of Mechanical
Engineering, Chengde Petroleum College, Chengde 067000, P. R. China)
Abstract: The FDM numerical simulation software, ViewCast system, was employed to simulate the low
pressure die casting (LPDC) of an aluminum wheel. By analyzing the mold-filling and solidification stage of the
LPDC process, the distribution of liquid fraction, temperature field and solidification pattern of castings were
studied. The potential shrinkage defects were predicted to be formed at the rim/spoke junctions, which is in
consistence with the X-ray detection result. The distribution pattern of the defects has also been studied. A solution
towards reducing such defects has been presented. The cooling capacity of the mold was improved by installing
water pipes both in the side mold and the top mold. Analysis on the shrinkage defects under forced cooling mode
proved that adding the cooling system in the mold is an effective method for reduction of shrinkage defects.
Key words: numerical simulation; aluminum wheel; low pressure die casting; defect reduction; forced cooling
CLC number: TG146.2+1/TP391.9 Document code: A Article ID: 1672-6421(2009)01-048-05
T he automotive industry is moving towards expanding
the application of light-weight aluminum alloy castings
for various components that previously made from steels or
pressure, leading to compact components with roughly 10%
improvement in their mechanical properties compared with
those manufactured by conventional foundry techniques [4-8].
cast irons, for example, more than 50% of new cars in North To reduce the cost, it is necessary to identify the hard-to-
America are now equipped with aluminum alloy wheels [1]. cast wheels prior to the die manufacture and prototyping, so
Unfortunately, because of the multiple stringent requirements that castability issues can be explored early in the designing
for surface finish, impact and fatigue performance, air stage to avoid the manufacture of faulty and expensive dies.
tightness, geometric and rotational balance tolerances, To take such advantages, the die casting industry has now
aluminum alloy die-cast wheels are one of the most difficult adopted the computer based simulation technique which
castings to make in automotive and the rejection rates are often showed advantages over the conventional trial-and-error
high compared with other aluminum castings [2]. methodologies for design and optimization [9-13]. In this paper,
Low-pressure die casting (LPDC) process is a near net shape the numerical simulation system ViewCast was used to
casting method [3]. Due to the high precision and high efficiency simulate the temperature and velocity fields during the filling
requirements as well as its capacity for high quality wheels and solidification stages of LPDC aluminum alloy wheel. The
at low cost LPDC is considered as the dominant process for present work was aimed to predict the location and volume
the production of aluminum alloy wheels. LPDC is a cyclic of defects, and modification has been applied to the original
process, which begins with the pressurization of the furnace. process in order to improve the quality of casting.
The high pressure inside the furnace forces the aluminum
melt to rise up and enter the die cavity where it solidifies 1 Experiment
by transferring the heat from the metal to the die. In LPDC
process the melt is poured from the bottom of the mould, 1.1 Mathematical model
thus the mould-filling course is smooth and can be regulated The flow of liquid metal was assumed to be incompressible
easily. Meanwhile, castings are solidified under external Newtonian fluid and the governing equations at the LPDC
filling and solidification stages are as follows:
*Mi Guofa Navier-stokes equation
Male, born in 1966, professor, Ph.D graduated from the Harbin
Institute of Technology (HIT) in 1989 and majored in Foundry, (1)
and he got his master’s degree and doctor’s degree from HIT in
1992 and 1995, respectively. His research interests are mainly
(2)
focus on metal solidification technology and new materials.
E-mail: peter@hpu.edu.cn
Received: 2008-06-26; Accepted: 2008-08-13 (3)
48
Research & Development
February 2009
Continuity equation Table 1 Material properties of A356
Material properties Values
(4)
2.485×10-3 (g/mm3, for solid)
Density
Heat-transfer equation 2.415×10-3 (g/mm3, for liquid)
Viscosity 2.96×10-3 (g/(mm •s))
(5)
Specific heat 0.963 (J/(g •℃))
Where t is the density; u, v and w, the velocity vectors; t, the Thermal conductivity 0.151 (W/(mm •℃))
time; µ, the dynamic viscosity of the liquid metal; gx, gy and Latent heat 389 (J/g)
gz, the gravitational acceleration vectors; p, the pressure; Cp, Liquidus temp. 615 ℃
the specific heat of molten metal; m, the thermal conductivity; Solidus temp. 555 ℃
T, the temperature; L, the latent heat and fs, the solid phase Gravity 9,800 (mm/ s2)
fraction at the solidification stage.
Table 2 Material properties of H13 steel
1.2 Geometric model
Material properties Values
Figure 1 shows the geometric model of aluminum alloy
Density 7.8×10-3 (g/mm3)
wheel, which was imported into the ViewCast. It consists of
Specific heat 0.422 (J/(g •℃))
10,000,000 FDM meshes, as shown in Fig. 2. The step length
Thermal conductivity 0.0287 (W/(mm •℃))
of mesh generation was self-adjusting to ensure the thinnest
part of the casting can be divided into three meshes and ensure pressure of fluid at the ingate; Vv is velocity of fluid surface in
the accuracy of flow simulation. the crucible; Pv is the vapor pressure above fluid; and h is the
height of ingate above the reservoir.
If we assume that the fluid velocity is relatively low (i.e.
the dynamic pressure is negligible) and that the volume of the
metal reservoir is substantially larger than the volume of the
casting (i.e. the height of fluid is constant), then the ingate
pressure can be expressed as a function of the applied air
pressure minus the hydrostatic head of molten metal.
(7)
Although we have assumed that the height of the fluid is
constant during the filling of each casting, this is not true from
Fig. 1 3D model of aluminum alloy wheel one casting to the next. As molten metal is consumed, the
volume of fluid in the crucible will be reduced progressively
and the ingate height will increase. Therefore, in theory, the
ingate height should be measured before every simulation, but
in practice, it is not necessary because the fluid is driven by the
increase rate of the applied pressure, rather than the absolute
pressure. Since the rate of increase is constant during the filling
process of the casting, the height of the riser-tube is irrelevant.
Based on the above analysis, the effect of molten metal’s
reduction in the crucible was neglected and the calculated
Fig. 2 The mesh of aluminum alloy wheel pressure at the gate, at a pouring temperature of 700℃ and a
mold temperature of 300℃, is listed in Fig. 3.
1.3 Initial and boundary conditions
The wheel was cast from Sr-modified A356 (Al-7%Si-0.3%Mg)
alloy and the mold material was H13 steel. The properties
of the alloy and mold were listed in Table 1 and Table 2,
respectively.
The pressure boundary condition can only be used if the
pressure at the gate is known. The pressure applied on the
liquid surface in the crucible was known in advance and the
pressure at the ingate can be calculated with the Bernoulli
equation
(6)
Where Vi is the velocity of fluid at the ingate; Pi refers to the Fig. 3 The LPDC pressure curve
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CHINA FOUNDRY Vol.6 No.1
2 Simulation results and discussion results are shown in Fig. 4. It indicates the positions of the
molten aluminum front in the cavity at different filling time.
The mold-filling analysis was conducted on an aluminum
The whole filling process takes 16.9 s.
wheel produced by the LPDC process, and the simulation
Fig. 4 Temperature distribution during mold-filling stage
At 1.1 s, the molten metal flows through the running
system into the die cavity and then fills the centre of the hub.
At 2.5 s, the front reaches the junctions between the rim
and the spokes, and fills the bottom of the rim at 4 s. At this
moment, the temperature of the alloy is about 600℃. The
liquid metal reaches the middle of the rim at 13.34 s with a
temperature of about 590℃, and reaches the top of the rim at
16.9 s. Figure 4 shows that the filling behavior is stable and
propitious to prevent the formation of gas entrapment during
the filling stage. When the cavity is filled entirely, no misrun
and cold-shut are found at the top of the rim. At this time, the
temperature of the front metal is about 580℃, which is in the Fig. 6 Temperature curves of the monitored points
during filling
semisolid state. In order to measure the temperature difference
between the top and the bottom of the rim, three monitors were
fixed at the middle of the rim, at the rim/spoke junction and Once the mold cavity is completely filled, solidification
at the top of the rim, respectively, as shown in Fig. 5. Figure simulation is followed. The filling simulation results should be
6 reveals the temperature evolution during the filling stage at used as the initial temperature distribution of the solidification
these positions. simulation for accuracy. Solidification is completed at 192 s.
Figure 7 shows the solidification time at critical regions.
Figures 7(f) and (g) shown that solidification at the spokes,
which begins at 53 s and finishes at 59 s, is faster than that
at other positions. As a result, some isolated liquid regions
(liquid islands) form at the rim/spoke junctions and eventually
leading to shrinkages, as shown in Fig. 8. Figure 9 is the result
of X-ray detection, exhibiting good agreement between the
simulation and the practical measurement, with high accuracy
for the predicted volume and position of the shrinkage.
During solidification stage, the temperature at the rim/spoke
junction is higher than that at the middle rim and top rim, as
shown in Fig. 10. Consequently, hot spots (liquid islands) at
Fig. 5 Positions of monitors
the rim/spoke junctions result (Fig. 7g), leading to potential
shrinkages at the final stage of the solidification. Due to
50
Research & Development
February 2009
217
167
(a) 21 s (b) 25 s (c) 29 s (d) 33 s
117
67.1
(a) 39 s (b) 53 s (c) 59 s (d) 69 s 17.0
Fig. 7 Solidification time at critical regions
the above reasons, the entire solidification process is not a
directional solidification pattern from the rim top towards the
hub center.
3 Defects reduction
Above analysis suggests that the hot spots occur at the rim/
spoke junctions. In order to prevent the formation of shrinkage
defects, the cooling capacity of the mold was enhanced by
adding cooling water pipes at both the top mold and the side
Fig. 8 The predicted shrinkage defects mold, as shown in Fig. 11. The diameter of the water pipe is
20 mm, and the temperature of the cooling water is 20℃.
1-Top mold 2-Side mold 3-Casting
4-Lower mold 5-Water pipes
Fig. 9 Result of X-ray detection
Fig. 11 Sketch showing the location of cooling
water pipes
Figure 12 displays the solidification sequence of the
wheel under forced-cooling. It showed that the time for the
generation of liquid islands is about 44 s, which is brought
forward by about 15 s when compared with the result obtained
by using of the mold without forced-cooling. The liquid
islands disappeared about 18 s ahead of the original process.
In comparison of Fig. 7 with Fig. 12, it can be seen that the
volume of the liquid island was decreased, however their
locations remain unchanged. We shall study and optimize the
process to shift the liquid islands in the center of the wheel
in the future. Figure 13 shows the simulation result of defect
Fig. 10 Temperature curves of the monitored points
during solidification under forced-cooling.
51
CHINA FOUNDRY Vol.6 No.1
178 According to the
above analysis, it seems
that defects that caused
by the structure of the
138
casting can be reduced
to a certain extent
(a) 16.91 s (b) 18.52 s (c) 20.13 s (d) 23.36 s
but cannot be totally
97.5
eliminated. Defects are
hard to be eliminated
without structural
57.2
modification of the
casting. This is also
in line with the actual
(a) 26.58 s (b) 37.87 s (c) 44.32 s (d) 51.12 s 16.9
results [14].
Fig. 12 Solidification time at critical regions under forced-cooling
57(11): 36-43.
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The present work is funded by the Innovation Fund for Outstanding Scholar of Henan Province (No.0621000700)
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